Double-focusing mass spectrometer having electrically adjustable electrostatic an alyzer and adjustable electrostatic lens



Feb. 1966 C. E DOUBLE-FOCUSING MASS SPECTROMETER HAVING ELECTRICALLY :www

. BERRY ET AL ADJUSTABLE ELECTROSTATIC ANALYZER AND ADJUSTABLE ELECTROSTATIC LENS Filed Sept. 16, 1965 fl/Tl United States Patent 3,233,099 DOUBLE-FQCUSING MASS SPEC'IROMETER HAVING ELECTRlCAL-LY ADJUSTABLE ELECTRSTATIC ANALYZER AND ADJUST- ABLE ELECTRGSTATIC LENS Clitord E. Berry, Aitadena, and Louis B. Becker, Pasadena, Calif., assignors to Consolidated Electrodynamcs Corporation, Pasadena, Caiif., a corporation of California Filed Sept. 16, 1963, Ser. No. 309,037

3 Claims. (Cl. Z50-41.9)

The present invention relates to mass analyzing or resolving systems and, more particularly, to improvements in double-focussing mass spectrometers.

-Double-focussing mass spectrometers are well known in the art and have, for example, been described in Mass Spectrometry and Its Applications to Organic Chemistry, by J. H. Beynon, Elsevier Publishing Cornpany and D. Van Nostrand Company, Inc., 1960, Chapter-I', Section 1.7; Zeitschrift fr Physik, 98, 1934, page 786, Mattauch and Herzog; and U.S. Patent 2,851,608, Mass Spectrometer, September 9, 1958, by Dr. C. F. Robinson. Briefly, a double-focussing mass spectrometer comprises means for causing the release of particles in ionized form from the substance under investigation, an electrostatic ion deector, a magnetic ion deflector and suitable ion sensing means.

It is also known in the art that some of the ions in the ion beam produced in this type of apparatus from the substance Vbeing investigated do not follow the optical axis of the spectrometer but emanate from the substance atvarious angles to this axis. The ions that are able to reach the recording or sensing medium of the spectrometer despite their angular deviation form so-called angular images. In addition, it is known that even those ions that travel along the optical axis leave the substance under investigation at dilTe-rent velocities and, as a result, form on the recording or sensing medium of' the spectrometer so-called velocity images.

A`Iritheory, a mass spectrometercan be built in which the velocity andv angular images coincide on the ion recording or sensing means. In practice, however, it has beenfioundl that this goal is difficult to realize with conventional mass spectrometer apparatus. In consequence, the resulting mass spectra contain undesirably broadened images for oneand the same substance, of which the first orderfangular images and the lirst order velocity images may be particularly bothersome, especially in cases, such as isotope work, where the spectrum lines of the surbstance components are rather closely spaced.

In the past, various mechanical adjustments have been proposed and effected in an attempt to reduce the abovementioned image broadening. However, the adjustments sofar proposed are generally inconvenient and .do not ordinarily yield the desired result.. Y

The subject'invention overcomes these disadvantages by providing convenient and effective means for bringing at least first order angular and velocity images into substan-tialcoincidence with each other and with the photographic plate or other ion detecting means of the mass spectrometer. The invention stems from the discovery that angular and velocity images will move into coincidence with each other when the potential with respect to ground of at least one of the two detiector plates of the electrostatic vdeflector assembly employed in double-focussing mass spectrometers is varied, or when -the potential at one deector plate isy varied in one sense and the potential at the Iother deflector plate in the opposite sense. We have discovered that these potential variations will bring about a coincidence between angular and velocity images, particularly those of the first order. Nevertheless, the point of coincidence of these angular and velocity images is not necessarily located at the photographic plate or other ion sensing means employed in doublefocussing mass spectrometers. There exists, therefore, a need for a means which will cause the joint movement of the substantially coinciding angular and velocity images onto the photographic plate or ion sensing means. We have found that an electrostatic lens or lens system combined with a source of variable potential constitutes such a means. The electrostatic lens has the property of moving angular and velocity images at similar speeds when the lens potential is varied. In this fashion, it is possible to move the angula-r and velocity images, which have been brought into substantial coincidence in the manner mentioned above, toward and onto the ion recording medium or sensing means.- This is the case even if the lens adjustment should cause a slightly different movement of the angular and velocity images, since such a diierence can be corrected by readjustment of the deflector plate Ipotentials.

In accorda-nce with a preferred embodiment of the invention, the mass spectrometer apparatus comprises means for sensing ions and deflector means including an electrostatic deflector having two spaced deflector plates. The apparatus includes means for applying to one of the deflector plates a rst potential with respect to ground and to the other detiector plate a second potential with respect to ground. The apparatus also has means for producing from the substances under investigation ions that form angular and velocity images in the vicinity of the ion sensing means.

. with each other, and an adjustable electrostatic lens means for moving the substantially coinciding angular and velocity images into coincidence with the photographic plate or other ion sensing means.

Apart from a photographic plate or ilm, the abovementioned means for sensing ions may comprise electrical sensing means. The first and second potential of the electrostatic deflector plates may be supplied by one high voltage supply that has an impedance or resistor with grounded center tap associated therewith. The means for varying at least one of the rst and second detlector plate potentials may comprise 4a variable voltage source tha-t is connected to add its output voltage to the plate-to-ground potential of one of the pla-tes.

More conveniently, the potential variation taught herein may be effected by varying the impedance or resistance in at least one leg of the above-mentioned center-grounded impedance or resistance. A particularly advantageous adjustment is also provided by means which subtract a variable voltage from the p'late-to-ground potential of one of the detiector pla-tes, while adding voltages to the plate-to-ground potential of the other deflector plate. In a preferred embodiment of the invention, this eiect is brought about by connecting across the above-mentioned high voltage supply a potentiometer with a grounded movable arm. The plates are connected to the stationary terminals of this potentiometer. If the movable arm of the potentiometer is located so as to divide the potentiometer into two resistor sections of equal value, there are equal, though oppositely poled, plate-to-ground potentials at the deilector plates. If the potentiometer arm is moved from its center position, the plate-to-ground potentials at the two deector plates vary in opposite senses. In addition, the apparatus includes electrostatic lens means for moving the substantially coinciding angular and velocity images into substantial coincidence with `the ion recording or sensing means.

The invention will become more readily apparent from the following detailed description of a prior art system and of embodiments of the inventon, illustrated by way f example in the accompanying drawings, in which:

FIG. l is a diagrammatic side View of a prior-art mass spectrometer;

FIG. 2 shows a detail of the apparatus shown in FIG. 1 wlth ion trajectories; and

FIG. 3 is a diagrammatic side View of an apparatus embodying the invention.

The apparatus shown in FIG. l will be recognized by those skilled in the art as being a double-focusing mass spectrometer of the so-called Mattauch type as described, for example, in the initially mentioned literature. This apparatus has a pair of spark electrodes 11 and 12 connected to a conventional Tesla transformer 14. It also includes an apertured accelerating electrode 1S which is.

ply y18 is grounded and the other terminal 21 is connectedto the Tesla transformer 14 so that the accelerating electrode 15 is suitably biased with respect to the spark electrodes 11 and 12.

The apparatus shown in FIG. 1 further includes the conventional electrostatic deflector 23 composed of de* ector plates 24 and 25 and the conventional magnetic deliector 26 composed of a pair ot magnetic pole pieces only one of which is shown in FIG. l at 27. Grounded apertured electrodes 29 and 39 are located between accelerating electrode 1S and detiector 23, and grounded apertured electrodes 31 and 32 are located between electrostatic deiiector Z3 and magnetic dellector 26 in a manner well known in the art. An ion sensing means 34, such as a photographic plate, is located at the magnetic deilector 26 for producing the mass spectrum. If desired, other ion sensing means, such as Faraday cups orl electron multiplier assemblies may be used in lieu of plate 34. The deector plates 24 and .25 of the electrostatic deector 23 are connected, respectively, to terminals 35 and 36 of a deflecting voltagel supply 37. A pair of series-connected resistors 3S and 39 is connected across terminals 35 and 36. The junction 4d between resistors 38 and 3% is grounded.

The structure shown in FIG. l and its operation are well known in the art. Initially, a sample 410i the substance to be tested is placed onto one of thespark electrodes 11 and 12. The system is then evacuated. Neither the vacuum envelope nor the evacuation apparatus have ben shown in FIG. l, as these means are well known in the art. During operation of the apparatus of FIG. 1, the potential supplied by Tesla transformer` of the mass spectrometer. However, it is well known thatl not all ions travel exactly along the optical axis 44. This fact has been schematically illustrated in FIG. 2 which shows an arrow 46 representing ions that travel rom'the sample 41 along the optical axis 44 at a tirst` velocity, an arrow 47 representing ions that travel along the optical axis 44 at a second, diierent velocity, and an arrow 48 representing ions that travel from sample 41 :at an angle a with respect to the optical axis 44. Ions of the type represented by arrows 46 and 47 form socalled velocity images, while ions of the type represented by arrow 4S form so-called angular images.

For the purpose of illustration, a velocity image has been shown in FIG. l at 58 and angular image at 5?. The images 5S and 59 neither coincide with each other nor with the emulsion surface of photographic plate 34. The visible images on the photographic plate are thus unduly broad. In the interest of a precise analysis, it

4 is, of course, highly desirable that the velocity and angular images meet'each other substantially on the photographic plate 34.

FIG. 3 shows means for realizing this desiderat-um. The basc parts of the mass spectrometer illustrated in FIG. 3 have been shown in FIG. l, so that thereference numerals used in FIG. l are employed in FIG. 3vfor denoting these parts. According to the invention in the embodiment shown in FIG. 3, the resistors 38 and 39 of the apapratus of FIG. 1 are replaced by a potentiometer 60 that is connected between terminals 35 and 36 of deliecting voltage supply 37 and has a grounded movable arm 62. If the movable arm 62 is located in its center position so that it divides the potentiometer 6i) into two resistor sections of equal resistance, the elect of the potentiometer 60 is the same asof the resistors 38 land 39 in FIG. 1. In this case, thevel'oclty and angular images normally fail to coincide with each other. We have discovered that these images move Lby different distances if an offset voltage AV is introduced at the electrical midpoint between deiiecting sectors 24 and 25, as has been mentioned above. VIn FIG. 3, this introductionof offset voltage AV is conveniently effected by moving the arm 62 of potentiometer 6). This is illustrated in FIG. 3 by dotted arm 62. In this manner, oneof the plates 24 and.25 wil experience an increase of AV in its potential with respect to ground, while the other plate will experience a decrease of AV in its potential with respect to ground. By adjusting these voltages AV, the velocity and angular images can be brought into mutual coincidence. However, this point of coincidence is not necessarilyl'ocate'd on plate 34. A further adustment is, therefore, desirable which moves this point of coincidence onto plate 34. In the embodiment in FIG. 3, this further adjustment is effected by an electric lens 65 which is located between deiiector systems 23 and 26 and is composed of a pair of grounded, apertured electrodes 66 and 67 having a further apertured electrode 68 disposed therebetween. The lens electrodeV 68 is connected to the .movable arm 70 of a unilaterally grounded potentiometer 71. A lens bias voltage source 72 that supplies a direcucurrent voltage of, say, one hundred volts is connected to potentiometer 71. If desired, the lens 65 may'also beoi a conventonal cylindrical type (not shown). Adjustment of potentiometer 71 will cause movement of the velocity and angular images relative to plate 34.` In this manner, it is not only possible to bring the velocity and angular images into mutual coincidence, but also substantially into concidence on', plate` 34. The amount of movement of the angular image in response to adjustment of potentiometer 71 will usually be somewhat different from the amount of simultaneous movement of the velocity image. However, this diiierence can largely be corrected by adjustment of potentiometer 6%). Since electrostatic lens systems are of the electrooptically positive type, it is avisable to locate the spark electrodes 11 and 12 slightly farther away` from the electrostatic deiiector 23' than in the prior art apparatus illustrated in FIG. l. The resulting coincidence of the above-mentioned images 59 and 58 on plate 34 is apparent from FIG. 3.

It will now be recognized that the invention provides a' particularly advantageous, convenient and effective means for obtaining high-quality operation of lrnass spectrometers,

AVarious modifications within the scope of the invention will be apparent to those skilled in the art.

We claim:

1. In a double-focusing mass spectrometer having in serial arrangement an ion source, including means for producing ions that orrnrst order angular and velocity images, an electrostatic `analyzing sector adjacent the ion source, the electrostatic sector having two spaced detlector plates, a magnetic analyzing sector, a drift space between the electro-static and magnetic sectors, and

means for sensing ions, the improvement comprising means for applying a first potential with respect to ground to one of the deilector plates of the electrostatic analyzing sector and a second potential with respect to the ground to the other detlector plates of the electrostatic analyzing sector, means for varying at least one of the first and second potentials to move the angular and velocity image-s substantially into coincidence with each other in the vicinity of the ion sensing means, and adjustable electrostatic lens means interposed in the drift space between the electrostatic analyzing sector and the magnetic analyzing sector for moving the substantially coinciding angular and velocity images onto the ion sensing means.

2. In a double-focusing mass spectrometer having in serial arrangement a source of ions of substances to be investigated including means for producing from said substances Iions that form images including a primary image, rst order angular images and iirst order velocity images, detlector means including an electrostatic deiector having two spaced deector plates located adjacent the source of ions and a magnetostatic deector, a drift space between the electrostatic and magnetostatic deectors and a collector for the ions, the improvement comprising means for applying to one of the deiiector plates of the electrostatic deector a rst potential with respect to ground and to the other detiector plate of the electrostatic deflector a second potential with respect to ground, means for reducing one of the rst and second potentials and for increasing the other of the first and second potentials to move the angular and velo-city images substantially into coincidence with each other in the vicinity of the collector, and an adjustable electrostatic lens located in the drift space between the electrostatic and magnetostatic deector for moving the substantially coinciding angular and velocity images into coincidence with the primary image at the collector.

3. ln a Mattauch-Herzog type of double-focusing mass spectrometer comprising an electrostatic ion deilector having two spaced delector plates, a magnetic ion detlector, means for sensing ions traversing a path through the electrostatic and magnetic ion deectors and an ion sour-ce for producing ions that form images including a primary image, first order angular images, and tirst order velocity images in the vicinity of the ion-sensing means, the improvement comprising a potentiometer having a grounded movable arm connected to the dellector plates of the electrostatic ion deflector, a deilection voltage supply connected across the potentiometer, an electrostatic lens located between the electrostatic ion deliector and magnetic ion deilector, and means for applying a variable electric potential to the electrostatic lens whereby adjustment of the potentiometer arm moves the angular and velocity images substantially into coincidence with each other and adjustment of the variable electric potential applied to the leus moves the lsubstantially coinciding angular and velocity images into coincidence with the primary image at the ion sensing means.

References Cited by the Examiner UNITED STATES PATENTS 2,533,966 12/1950 Simmons Z50-41.9 2,629,056 2/1953 Goodwin Z50- 41.9 2,851,608 9/1958 Robinson Z50- 41.9 2,908,816 l0/1959 Leloole 2SC-41.9 2,957,985 10/1960 Brubaker Z50-41.9 3,010,017 11/1961 Brubaker et al. Z50- 41.9

FQRElGN PATENTS 1,115,501 4/1956 France.

f RALPH G. NILSON, Primary Examiner. 

1. IN A DOUBLE-FOCUSING MASS SPECTROMETER HAVING IN SERIAL ARRANGEMENT AN ION SOURCE, INCLUDING MEANS FOR PRODUCING IONS THAT FORM FIRST ORDER ANGULAR AND VELOCITY IMAGES, AN ELECTROSTATIC ANALYZING SECTOR ADJACENT THE ION SOURCE, THE ELECTROSTATIC SECTOR HAVING TWO SPACED DEFLECTOR PLATES, A MAGNETIC ANALYZING SECTOR, A DRIFT SPACE BETWEEN THE ELECTROSTATIC AND MAGNETIC SECTORS, AND MEANS FOR SENSING IONS, THE IMPROVEMENT COMPRISING MEANS FOR APPLYING A FIRST POTENTIAL WITH RESPECT TO GROUND TO ONE OF THE DEFLECTOR PLATES OF THE ELECTROSTATIC ANALYZING SECTOR AND A SECOND POTENTIAL WITH RESPECT TO THE GROUND TO THE OTHER DEFLECTOR PLATES OF THE ELECTROSTATIC ANALYZING SECTOR, MEANS FOR VARYING AT LEAST ONE OF THE FIRST AND SECOND POTENTIALS TO MOVE THE ANGULAR AND VELOCITY IMAGES SUBSTANTIALLY INTO COINCIDENCE WITH EACH OTHER IN THE VICINITY OF THE ION SENSING MEANS, AND ADJUSTABLE ELECTROSTATIC LENS MEANS INTERPOSED IN THE DRIFT 